1. Field of the Invention
The present invention relates to a discrete time control apparatus for controlling a system with continuous time, and more particularly to a discrete time control apparatus having a discrete-time adaptive switching control capability and a continuous manipulated-variable sampling control capability.
2. Description of the Prior Art
PID (Proportional, plus Integral plus Derivative) controls are widely employed for controlling the processes in plastics processing, industrial furnaces, chemical plants, and the like. In order to control systems properly with such PID controls, it is necessary to establish PID parameters accurately. The tuning of PID parameters has been dependent largely on the experience of operators involved in the control practice, and it has been difficult for the operators to set appropriate PID parameters when the characteristics of the controlled systems vary significantly. To eliminate such difficulty, there has heretofore been proposed an automatic tuning PID control apparatus employing the step response method or the ultimate sensitivity method. With such an automatic PID control apparatus, however, it is necessary to operate the controlled system in advance for parameter tuning, and difficulty has been experienced in achieving desired response at the starting time of operating the controlled system, changing a preset value and achieving the stability in a normal operating condition.
There has also been proposed a discrete-time adaptive switching control (hereinafter referred to as "ASC") apparatus using an inexpensive and rugged binary switching actuator for actuating a controlled system with a continuous quantity (Japanese Patent Application No. 59-113781). It is known that the ASC apparatus is much better than conventional PID controllers with respect to operation starting characteristics and settling ability against disturbance in controlled systems having a relatively large time constant, such as temperature control systems. However, the ASC apparatus has the disadvantage that the sampling period cannot be greatly reduced, since the manipulated variables are of two values, i.e., ON and OFF states, and a large amount of arithmetic operation is required. As a result, the controlled variable varies to a larger extent under normal operating condition than general sampling control apparatus employing continuous manipulated variables.